I get:
$ desktop-file-validate /usr/share/applications/balsa-mailto-handler.desktop
/usr/share/applications/balsa-mailto-handler.desktop: error: value "@TOOLKIT_CATEGORIES@Network;Email;" for key "Categories" in group "Desktop Entry" contains an unregistered value "@TOOLKIT_CATEGORIES@Network"; values extending the format should start with "X-"
/usr/share/applications/balsa-mailto-handler.desktop: hint: value item "Email" in key "Categories" in group "Desktop Entry" can be extended with another category among the following categories: Office, or Network
/usr/share/applications/balsa-mailto-handler.desktop: hint: value "@TOOLKIT_CATEGORIES@Network;Email;" for key "Categories" in group "Desktop Entry" does not contain a registered main category; application might only show up in a "catch-all" section of the application menu

And, indeed, it looks something wasn't replaced as expected:
$ cat /usr/share/applications/balsa-mailto-handler.desktop
[Desktop Entry]
Name=Balsa
Name[es]=Balsa
Comment=E-Mail utility
Comment[es]=Utilidad de correo-e
GenericName=Mail Client
X-GNOME-FullName=Balsa Mail Client
TryExec=balsa
Exec=balsa -m %U
Icon=gnome-balsa2
Terminal=false
Type=Application
StartupNotify=true
Categories=@TOOLKIT_CATEGORIES@Network;Email;
MimeType=x-scheme-handler/mailto;
NoDisplay=true